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Cassini Significant Events 4/5/17—4/11/17

Two men at computer consoles.
Science planners Sherwin Goo, left, and Rudy Boehmer begin the process to send the commands that will launch Cassini’s Grand Finale. It will take the commands 80 minutes to travel from Earth to the spacecraft at Saturn once they are relayed via NASA’s Deep Space Network. Once Cassini receives the data, it will store the commands, and begin executing them on April 14, in time to begin the Grand Finale on April 26, 2017.

Cassini's orbits about Saturn are adjusted, using gravity assist, when the spacecraft passes by Saturn's largest moon, Titan. The shape of each new orbit fits in with the overall years-long plan for the mission. Titan by Titan, the orbits' heights and inclinations and periods conform right to plan over the years. Only with such a stable plan can scientists determine in advance exactly what observations they can make, and exactly when.

Flying by Titan again on April 22 is inevitable. Even if the spacecraft were to unexpectedly die, the T-126 targeted encounter will happen on that day, and Cassini's orbit will change again; the laws of Sir Isaac Newton are at the helm. Cassini will interact with Titan next week, and then will impinge on Saturn's atmosphere on September 15, no matter what.

On Tuesday this week, the flight team sent a new sequence of commands, S99, which will control the spacecraft's science and engineering activities spanning its first Proximal orbit – the first flight between Saturn's atmosphere and its innermost rings – and five more such orbits after that. Command Sequences S100 and S101 will then continue to see Cassini's activities through the rest of the 22 Proximal orbits and Grand Finale. A new video on this page captures the live commanding process at JPL during the S99 uplink: https://go.nasa.gov/2oq3jnM.

Wednesday, April 5 (DOY 095)

Cassini was busy today with the weekly ring-plane crossing near periapsis, number 18 out of the 20 planned F-ring grazing orbits.

The bright red star Gamma Crucis (the "top" of the Southern Cross) drifted behind Saturn's atmosphere thanks to Cassini's motion. The Visible and Infrared Mapping Spectrometer (VIMS) watched the star for 2.1 hours, in collaboration with the Composite Infrared Spectrometer (CIRS), to complete measurements that were started on April 4, to determine the abundance of helium in Saturn's atmosphere.

Next, the Ultraviolet Imaging Spectrograph (UVIS) trained its detectors on the bright blue star Alpha Eridani as it slipped behind Saturn's icy moon Dione, which is 1,128 kilometers wide. This 1.2-hour stellar occultation helped search for any telltale signs of an atmosphere that might be present on that moon.

While the spacecraft sped towards periapsis, CIRS scanned Saturn's rings from directly above their sunlit side for 2.6 hours to make a high-resolution profile of the rings in the thermal infrared part of the spectrum. UVIS and VIMS rode along. The Imaging Science Subsystem (ISS) then trained its cameras on the A and B rings to create a radial mosaic at resolutions ranging between 0.4 and 1 km per pixel. All the other Optical Remote Sensing (ORS) instruments, CIRS, UVIS, and VIMS also observed.

As ring-plane crossing occurred some 6,000 km outside of the F ring core, the Ion Neutral and Mass Spectrometer (INMS) led a joint observation with Cassini's Cosmic Dust Analyzer (CDA) to make in-situ measurements in the vicinity of Saturn's F ring for the two hours closest to the ring. At the same time, the Radio and Plasma Wave Science (RPWS) instrument increased its sampling frequency, tuning in at high resolution to the plasma waves there, in the vicinity of the magnetic equator. It also and characterized the flux of dust impacting the spacecraft.

As the spacecraft started to swing outbound from Saturn, ISS again led the other ORS instruments in a three-hour scan across the now unlit side of the A and B rings, making a radial mosaic at the same resolution as on the lit side. Following this observation, ISS, CIRS, and UVIS targeted the outer edge of the A ring, plus the F ring, and the area in between, for four hours to study the structures and dynamics there at high spatial resolutions.

Finishing out this busy observing time while close to Saturn and the rings, ISS, CIRS, and UVIS tracked ring features known as propellers, for one hour. A 5.5-hour CIRS observation of the narrow trans-Encke region in Saturn's outer A ring capped off the day, with UVIS riding along. The moderate wavelength-resolution spectra taken during this observation will add to knowledge of the composition and structure of this unique region in Saturn's rings.

Cassini's website is in the running for a Webby Award:/news/13014/nasa-up-for-five-webby-awards.

Thursday, April 6 (DOY 096)

Six Deep Space Network (DSN) stations and one European Space Agency (ESA) station, spanning three continents, were trained on Cassini today. The reason? To capture and record Cassini's continuous radio signals at Ka-band (32 GHz) X-band (8 GHz) and S-band (2 GHz) after they had been affected by passing through Saturn's rings and atmosphere. The spacecraft's telemetry was turned off, to provide pure-tone radio signals for the duration of the seven-hour, high-resolution Radio Science occultation experiment.

A non-targeted flyby of Titan occurred late in the day. At closest approach, Titan took up about one-sixth of the ISS Wide Angle Camera's field of view. ISS spent 4.4 hours making two observations of Titan, with VIMS riding along. In between these, VIMS observed the red star Alpha Orionis – Betelgeuse – as it went behind Saturn's atmosphere. VIMS finished the day by tracking the full radial egress occultation of Betelgeuse, as Cassini's line of sight to the star cut across Saturn's main rings. Low-elevation occultations such as this help discern the structure of low-optical-depth regions in the rings. The viewing geometry is illustrated here, with background Betelgeuse beginning the 3.6-hour ring occultation.

Friday, April 7 (DOY 097)

Cassini turned to point the ORS instruments to Titan for 14.2 hours, while ISS, VIMS and CIRS took advantage of coming within 478,000 km of Saturn's planet-like moon.

RPWS ramped up its data-collection rate to better observe the auroral magnetosphere and the source regions of the low-frequency radio emissions known as the Saturn kilometric radiation (SKR).

For the final science observation of the day, ISS and CIRS spent 5.5 hours viewing Saturn's faint rings at high phase angle, a geometry favorable for viewing small dust particles in these structures.

A feature on the making of the brilliant 3.6-minute movie "Cassini's Grand Finale" was published today: /news/13016/making-cassinis-grand-finale.

Saturday, April 8 (DOY 098)

ISS and CIRS spent another 13.3 hours observing the faint rings at high phase. The goal of these observations was to monitor time-variable features in order to better understand the temporal and seasonal evolution of these dusty systems.

Sunday, April 9 (DOY 099)

Cassini coasted through apoapsis at the start of the day, marking the beginning of Orbit #268.

Monday, April 10 (DOY 100)

CIRS and UVIS observed the far-infrared reflection spectra from part of Saturn's B ring to constrain ring particle composition. This eight-hour observation helped compare the spectral properties of ices among different regions of Saturn's main rings, as well as its icy moons.

VIMS watched a slow ingress occultation of the red star VY Canis Majoris for 6.2 hours as it passed behind the narrow F ring and all the way inward to the strands of the C ring. Then at the end of the day, ISS and VIMS began a 90-minute Titan monitoring observation, watching its weather from 1.6 million km away.

Tuesday, April 11 (DOY 101)

UVIS, CIRS and VIMS performed a series of slews and stares across Saturn's northern auroral region. From this geometry, the optical instruments captured auroral dynamics across the full polar region while Magnetospheric and Plasma Science (MAPS) instruments directly measured the local plasma and magnetic field conditions.

The week's final observation was another stellar ring occultation. The exploding star Eta Carinae is one of the few point-sources bright enough at infrared wavelengths for CIRS to use. Such occultations provide a measure of the rings' optical depth at a unique region of the electromagnetic spectrum.

New science results about ocean worlds in our solar system will be the subject of a news conference to be held this Thursday April 13 at 2pm EDT:https://go.nasa.gov/2okbMqQ.

Watch the moon slide by golden Saturn from April 14th through the 17th. The best time to look are 2 am until dawn in the southern sky. Saturn is a stunning object to see, especially the way its rings are wide open towards Earth these days.

The DSN communicated with and tracked Cassini on thirteen occasions this week, using stations in Spain, California, and Australia. A total of 21,237 individual commands were uplinked, and about 2,137 megabytes of science and engineering telemetry data were downlinked and captured at rates as high as 124,426 bits per second.

Wrap up:

Cassini is executing its set of F-ring-grazing orbits of Saturn, with a period of 7.2 days in a plane inclined 63.6 degrees from the planet's equatorial plane. The 20 orbits are nearly identical, with Cassini's nearest point at about 150,000 kilometers, and farthest point at about 1.28 million km from Saturn. Speeds relative to Saturn at those points (periapsis and apoapsis), are close to 76,150 km per hour and 9,000 km/h respectively.

The most recent spacecraft tracking and telemetry data were obtained on April 11 using the 70-meter diameter DSN station in Australia. The spacecraft continues to be in an excellent state of health with all of its subsystems operating normally except for the instrument issues described at http://saturn.jpl.nasa.gov/anomalies.

Cassini's orbit looks the same again this week, but the positions of spacecraft and moons and the Sun are different, in this illustration of Cassini's path up to mid-day April 11: http://go.nasa.gov/2mKAXli.

The countdown clock in Mission Control shows 157 days until the end of the Mission. This page offers all the details of the Mission's ending: https://saturn.jpl.nasa.gov/mission/grand-finale/overview.

Information on the present position and speed of the Cassini spacecraft may be found on the "Present Position" page at:

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